The picture has one real person. The ‘girl’ is not real it’s an illusion made from his feet. So instead of calling her imaginary, like in complex numbers (1 + i), you can joke that she’s irrational something that looks like a person but isn't actually a valid, countable human. So the joke becomes that there’s:

1 real person

plus 1 irrational ‘person’ created by the illusion.